1. Field of the Invention
The invention disclosed and claimed herein generally pertains to an apparatus for providing fault protection, in a power supply or in a circuit that connects an electronic device to a power supply. More particularly, the invention pertains to apparatus of the above type wherein faults can occur that cause excessive levels of current to flow to the connection of the electronic device from the power supply. Even more particularly, the invention pertains to apparatus of the above type wherein one or more mobile or other electronic devices are removably insertable into a docking station, or otherwise connected to a power supply, in order to receive power.
2. Description of the Related Art
The use of electronic devices for computing and communications is becoming increasingly diverse, as the mobility of such devices increases and their costs diminish. As an example, mobile computing devices are currently being made available to customers at certain large discount stores. The mobile computing devices reside in a rack near a store entrance, and can easily be mounted on shopping carts by customers, to assist them in their shopping. When a device is returned to the rack, it is inserted into a charging or docking station, to allow the device battery to be recharged from a power supply.
It is very common for a mobile device to be connected to a docking station, with the power turned on to both the docking station and the device. Because of this, power supplies used to provide current for recharging mobile devices are typically provided with over-current sensor and shutdown circuits. Thus, if excessive current is drawn from the power supply, so that a threshold is exceeded, the circuit will operate to shut the power supply down. The threshold for the amount of current drawn that will cause shutdown is usually selected to be significantly more than the amount of current required for normal operation of the docking station. This is done so that shutdown will not occur falsely or erroneously. The threshold can in fact be so high that the connector pins, or other elements used to connect the mobile device to the docking station, can become pitted or otherwise damaged by faults that cause excessive current. Moreover, typical shutdown circuits pulse the power supply back on, in order to sense if the fault is still present, and if so, act to shut the power supply back down. This causes the power supply to pulse on and off, possibly for a long period of time, until the fault has been corrected.
Unfortunately, in highly public places such as stores of the above type, it is common for debris comprising conductive material, such as coins, tin foil or the like, to become scattered around the recharging rack that contains the mobile devices while not in use. It has been found that if debris of this sort comes into contact with a structure connecting a device to the charging station, the connecting structure may be short circuited to ground. Such a short circuit fault can cause an excessive amount of current to be drawn from the power supply. Moreover, a fault of this type tends to remain until corrected manually. Thus, if a short circuit fault occurs, a power supply provided with a current shutdown circuit as described above could continue to pulse on and off for a very long period of time, which is not desirable
A further problem in circuits used with charging or docking stations relates to inrush current. When a mobile device is connected to a charging station and the battery charge of the device is low, a substantial inrush current will initially flow into the charging station and the device, from the power supply. The inrush current can cause the connectors to pit, and can also drag down the voltage on the host machine to the point where the power supply of the host machine goes out of regulation. Accordingly, it is common to provide an inrush control circuit that includes power transistors. However, the combination of an inrush control circuit, together with an over-current protection circuit of the type described above, can create further problems. For example, if the over-current shutoff threshold is high enough to avoid false shutdowns, the resulting current may be high enough to damage the power transistors of the inrush control circuit. This is especially likely to happen with power supplies that pulse on and off for long periods of time, when too much current is being drawn. Under this condition the inrush current control circuit acts repeatedly, to try and limit the large current, and will overheat unless large transistors with heat sinks are used.
It would be desirable to provide an improved mechanism for protecting against excessive current in circuits that connect a power supply to a docking station. Such mechanism would be particularly useful in regard to excess currents that are caused by short circuit faults of the type described above.